A casing for electrical equipment, for example, a casing for an inverter is formed of a metal plate such as an iron plate. A louver structure 1 as shown in FIGS. 4 and 5 is generally employed when a drip-proof construction is required for openings formed in a side of the casing for the purpose of cooling an atmosphere in an interior of the casing. The structure is referred to as “louver window,” in which a metal plate 2 forming the casing is provided with outwardly protruding louvers 3 and openings 4 which are vertically arranged into a plurality of stages, respectively. Each one of the louvers 3 has an upper end and right and left ends all of which are continuous to the plate 2. Each louver 3 further has a lower end separated from the plate 2, so that an opening 4 is defined inside the lower end. The louver structure 1 thus constructed can ensure ventilation between an interior and an exterior of the casing through the openings 4 thereby to be capable of cooling an atmosphere in the interior of the casing. Further, the louvers 3 are constructed to prevent water drops from entering into the interior of the casing.
The louver structure 1 thus constructed has the following problems. The louvers 3 protrude outward from the side surface of the plate 2. This increases an overall size of product. Further, a forming die (a metal die) is required to obtain a shown shape of louver 3.